Here's the analysis for all warp cores of all races, and below an analysis of which empire leads in each category (note this information will most likely change after the patch):
Romulan Warp Cores
Average Mass / Output Efficiency 47.88
Average Cost /Output Efficiency 127.7
Average Ouput 66.5
Average Mass 3382.5
Average Cost 8492
Average Health 125.6
Federation Warp Cores
Average Mass / Output Efficiency 43.7
Average Cost /Output Efficiency 129.64
Average Ouput 65.8
Average Mass 3090
Average Cost 8530
Average Health 126.2
Klingon Warp Cores
Average Mass / Output Efficiency 47.29
Average Cost /Output Efficiency 137.57
Average Ouput 55.5
Average Mass 2800
Average Cost 7635
Average Health 106.2
Borg Warp Cores
Average Mass / Output Efficiency 49.03
Average Cost /Output Efficiency 153.29
Average Ouput 58.5
Average Mass 3090
Average Cost 8967.5
Average Health 336
MASS / OUTPUT EFFICIENCY (Lower Number is Better)
#1 Federation (for their mass, Fedration Warp cores produce the most energy)
#2 Klingon
#3 Romulan
#4 Borg
COST / OUTPUT EFFICIENCY (Lower Number is Better)
#1 Romulan (Romulan cores are the most cost effective, for the energy they produce)
#2 Federation
#3 Klingon
#4 Borg
OUTPUT (Higher Number is Better)
#1 Romulan (Romulan cores produce the most power overall)
#2 Federation
#3 Borg
#4 Klingon
MASS (Lower Number is Better)
#1 Klingon (Klingon cores are the lightest / least massive)
#2 Federation & Borg (Surprisingly, the Borg and Federation have the same mass warp cores overall)
#3 Romulan
COST (Lower Number is Better)
#1 Klingon (the Klingons check in with the least expensive warp cores)
#2 Romulan
#3 Federation
#4 Borg
HEALTH (Higher Number is Better)
#1 Borg (the Borg by far have the toughest warp cores -- I think they should be even tougher, though)
#2 Federation
#3 Romulan
#4 Klingon
It is interesting to see the Federation dominating only one category: Mass / Output Efficiency.
Romulans dominate two categories: Cost /Output Efficiency and Output
Klingons dominate in two categories, as well: Mass and Cost.
Borg dominate only one category: Health
If we weight everything equally, we can assign ranking points thus:
#1 = 4 points; #2 = 3 points; #3 = 2 points; #4 = 1 point
So, we obtain an overall ranking for all the empires' warp cores overall:
#1 Federation = 18
#2 Romulan = 17
#3 Klingon = 15
#4 Borg = 12
Therefore, the Federation has the best warp cores overall, in addition to the most mass / output efficiency. The fact that the Federation has the best core designers in the Alpha and Beta quadrants acconts for this advantage.
The Romulans with their quantum singularity cores come in a close second, main advantages being total output and cost effectiveness.
The Klingons are a distant third to the Federation, and almost equally behind the Romulans. When the Romulans developed their quantum singularity cores, this put them up over the klingons in terms of technology. The Klingons don't value science as do the Romulans and Federation, so this makes sense. Main advantages here are light, inexpensive cores.
The Borg don't use warp cores for travelling great distances. Therefore, they've put most of their assimilation efforts into improving transwarp. Meanwhile, they use their conventional warp core to power systems and travel short distances. So not very much assimilation effort is being put into improving this hold-over technology, that will eventually be replaced completely by transwarp. Also, these cores don't have the power drain of shields, making further development of warp technology much less important for the Borg than any other race.